Here's something interesting - because I majored in Religion in college, I learned Koine Greek so that I could read the New Testament from the source. And while that particular language has kind of slipped away from me since then (you try finding someone to speak Koine Greek with in the Midwest) I can still nail down a word or two that we've inherited in English.
One of these is a little piece of punctuation that everyone loves like a puppy with floppy ears - the apostrophe! And what does that word mean?
If you know anything about Greek plays (and why don't you, I ask?) you know that there was a word that applied to just about everything contained therein - for example, an ode is composed of a strophe, antistrophe, and an epode. And no, there won't be a quiz later. But if you ever walk up to me in public and reiterate any little piece of knowledge I bestowed on you during the WOLF I'll be totally flattered.
What does this have to do with apostrophe? A lot, I swear.
In Greek plays, an apostrophe was when the actor addressed someone who wasn't there, whether they be offstage or simply uh... not there. Is Hamlet's speech to Yorick technically an apostrophe? Um... yeah I'm not as smart as I pretend to be so you'll have to ask someone else that question.
So what do we use an apostrophe for in English? To smash up our words, of course. "Do not" becomes "don't" - and the apostrophe stands for... the "o" that's not there.
And while I know you're getting ready to blindside me with the ownership argument, (as in Mindy's pants) let me put it down in the ground with Yorick. Old English used "es" to denote ownership, and we dropped the pesky "e" and put in... the apostrophe to show that we went ahead and ditched the "e."